Acousto-optic modulator

ABSTRACT

According to one embodiment, an acousto-optic modulator includes an acousto-optic medium and a piezoelectric transducer. The acousto-optic medium has a configuration of a hexahedron. The acousto-optic medium has surfaces D, E, F, G and H. The piezoelectric transducer is provided on a surface C of the acousto-optic medium. The surface D opposes the surface C and has respective four sides shared by the surfaces E, F, G and H. Four angles defined between the surface D and the surfaces E, F, G and H each is other than 90°. At least one of eight angles defined between each pair of the surfaces C, E, F, G and H is other than 90°. The each pair has one shared side.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2011-206336, filed Sep. 21, 2011, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate to acousto-optic modulators capableof perform accurate optical modulation.

BACKGROUND

To realize accuracy and stability of optical modulation, it is importantto stabilize the luminous intensity of the traveling wave generated.However, it is known that since in general, an acousto-optic mediumsignificantly differs in acoustic impedance from air, ultrasonic wavesreflect on the boundary surface of the acousto-optic medium to degradethe stability of ultrasonic wave travelling in crystal. There is aconventional technique developed to avoid the degradation. In thistechnique, the surface of the acousto-optic medium opposed to a surfaceof a piezoelectric transducer and the surface of the piezoelectrictransducer are not parallel thereto to prevent direct reflection, and anultrasonic wave absorbing material is employed to absorb ultrasonicwaves.

However, such an absorbing material does not exist that has an acousticimpedance completely equal to that of the acousto-optic medium, canachieve sufficient ultrasonic wave attenuation, and can be easilyinstalled and produced at low cost. Accordingly, in the conventionalacousto-optic modulators, certain ultrasonic reflection waves exist,which may cause various types of degradation of modulation accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating an acousto-optic modulator according to anembodiment;

FIG. 2 is a view illustrating the configuration of an acousto-opticmedium according to the embodiment and a first example;

FIG. 3 is a view illustrating the acousto-optic medium of FIG. 2 seenfrom a direction perpendicular to surface H in FIG. 2;

FIG. 4 is a view illustrating an ultrasonic wave reflection modeemployed in the acousto-optic medium of FIG. 2;

FIG. 5 is a view illustrating the configuration of an acousto-opticmedium according to a second example;

FIG. 6 is a view illustrating an ultrasonic wave reflection modeemployed in the acousto-optic medium of FIG. 5;

FIG. 7 is a view illustrating the configuration of an acousto-opticmedium according to a third example;

FIG. 8 is a view illustrating an ultrasonic wave reflection modeemployed in the acousto-optic medium of FIG. 7; and

FIG. 9 is a view illustrating the configuration of an acousto-opticmedium according to a fourth example.

DETAILED DESCRIPTION

An acousto-optic modulator according to an embodiment will be describedin detail with reference to the accompanying drawings. In the embodimentand examples thereof, like reference numbers denote like elements, andno duplicate descriptions will be given of the elements.

The embodiment has been developed in view of the above-mentionedproblem, and aims to provide an acousto-optic modulator that achievesenhancement of stability in the intensity of light generated duringfrequency modulation.

According to one embodiment, an acousto-optic modulator includes anacousto-optic medium and a piezoelectric transducer. The acousto-opticmedium has a configuration of a hexahedron. The acousto-optic medium hassurfaces D, E, F, G and H. The piezoelectric transducer is provided on asurface C of the acousto-optic medium. The surface D opposes the surfaceC and has respective four sides shared by the surfaces E, F, G and H.Four angles defined between the surface D and the surfaces E, F, G and Heach is other than 90°. At least one of eight angles defined betweeneach pair of the surfaces C, E, F, G and H is other than 90°. The eachpair has one shared side.

The acousto-optic modulator is used as an apparatus for modulating thefrequency, intensity and direction of light. In general, theacousto-optic modulator comprises an acousto-optic medium 100, apiezoelectric transducer 110, and a high-frequency circuit 130, and iswidely used as an optical modulator. Alternatively, an apparatus thatcomprises the acousto-optic medium 100 and the piezoelectric transducer110, and does not comprise high-frequency circuit 130 may be called anacousto-optic modulator.

The acousto-optic medium 100 is a medium that propagates ultrasonicwaves and light, and is formed of single crystal or glass, for example,single crystal of tellurium dioxide or lead molybdate, or glasscontaining tellurium dioxide or lead molybdate.

The piezoelectric transducer 110 is adhered to the acousto-optic medium100 and is connected to the high-frequency circuit 130 via metal wires(e.g., copper wires 120).

The operation principle of the acousto-optic modulator will now bedescribed.

The high-frequency circuit 130 outputs a high-frequency signal to thepiezoelectric transducer 110, where the signal is converted intoultrasonic waves. The thus-generated ultrasonic waves serve as thetraveling wave and enter the acousto-optic medium 100 to thereby form apattern of condensation and rarefaction. The pattern serves as adiffraction grating for incident light, and the diffracted light ismodulated in accordance with the frequency and intensity of theultrasonic waves. This diffraction phenomenon is called an acousto-opticeffect.

In the conventional acousto-optic modulators, there is a mode in whichthe ultrasonic wave emitted by the piezoelectric transducer is reflectedseveral times by a plurality of surfaces of the acousto-optic medium100, perpendicularly enters a plane on which the piezoelectrictransducer is provided, and interferes another ultrasonic wave newlyemitted by the piezoelectric transducer. This mode is equivalent to astate in which the acousto-optic medium serves as a Fabry-Perot cavityfor ultrasonic waves. In the conventional acousto-optic modulators, aplurality of reflection modes occur in accordance with the times ofreflection of an ultrasonic wave on a plane opposing the plane with thepiezoelectric transducer provided thereon. In accordance with thereflection modes, a noise pattern of the intensity occurs, in whichrepetitive noise occurs for each of certain frequencies. The noisepattern of the intensity becomes an important problem when accurateoptical frequency modulation is performed, and there is a demand forimproving the same.

In view of the above, the acousto-optic modulator of the embodiment isdesigned to a configuration that is free from the above-mentioned modes.If the acousto-optic modulator is of a hexahedron, its configuration isset so that the surface thereof, to which the piezoelectric transducer110 is attached, does not permit light to perpendicularly enter it. Ifthe acousto-optic modulator is of a heptahedron or octahedron, etc.,other than the hexahedron, this is more advantageous in preventing lightfrom entering the surface of the element at right angles with respectthereto, to which surface the piezoelectric transducer 110 is adhered.

Referring then to FIGS. 2 and 3, the acousto-optic modulator of theembodiment will be described specifically. The acousto-optic modulatorof the embodiment has a configuration that has a function of suppressingthe noise pattern of the intensity occurring during frequencymodulation.

As described above, the acousto-optic modulator of the embodimentcomprises the acousto-optic medium 100, the piezoelectric transducer110, and a sound absorbing material 210. The sound absorbing material210 is, however, not indispensable.

As shown in FIG. 2, the surface on which the piezoelectric transducer110 is provided is defined as a surface C, the surface opposing thesurface C is defined as a surface D, one of the light input and outputsurfaces that has a smaller area than the other is defined as a surfaceE, the other surface having a larger area is defined as a surface F, thesurface other than the above-mentioned surfaces and covering the soundabsorbing material 210 is defined as a surface G, and the surfaceopposing the surface G is defined as a surface H. In the embodiment, thefour angles formed between the surface D opposing the surface C and thefour surfaces E, F, G and H (each of which shares its one side with thecorresponding one of the four sides of the surface D) are set to valuesother than 90°. Further, at least one of the eight angles formed betweeneach pair of the surfaces C, E, F, G and H (the each pair having oneshared side) is set to a value other than 90°.

In the acousto-optic modulator of the embodiment, the angle definedbetween the surfaces F and C is set to a value other than 90°, as shownin FIG. 2. Further, the angle defined between the surfaces F and C isset to θ′+π/2, and the angle defined between the surfaces C and D is setto θ. FIG. 3 is a view seen from above the surface H in a directionperpendicular to the surface H, and in this structure, the angle definedbetween the surfaces F and C is set greater than 90°. Further, unlessθ′=nθ/2 (n is an integer not lower than 1), a mode in which anultrasonic wave enters the surface C at right angles with respectthereto does not exist. In other words, a condition that the mode inwhich the ultrasonic wave enters the surface C at right angles does notexist means that no matter how many times the ultrasonic wave enters thesurface C, it does not enter the surface C at right angles. Thiscondition is a condition for preventing a cavity mode (in whichreflection of the same trajectory is iterated). Since ultrasonic wavesare output from the surface C at right angles with respect thereto, sucha mode as in which the ultrasonic waves pass the same trajectory doesnot exist unless they again enter the surface C at right angles.

The above condition can be derived based on the fact that the incidentangle with respect to the surface C is not 90° as shown in FIG. 4.Further, even if θ′=nθ/2 is satisfied, the number of times of reflectionof an ultrasonic wave in the acousto-optic medium is greater than in thecase where θ′=0, and therefore interference is weakened by theultrasonic wave absorption, of the sound absorbing material attached tothe acousto-optic medium or by attenuation of the ultrasonic wave in thecrystal, thereby suppressing the noise level.

The same effect as resulting from the configuration of FIG. 2 can beobtained by setting the angle of the surface H with respect to thesurface C to a value other than 90°, as is shown in FIG. 5 (see a secondexample described later), or by setting, to a value other than 90° withrespect to the other surfaces, the angle of the surface C itself onwhich the piezoelectric transducer is placed (see a third exampledescribed later). Furthermore, the interference mode can be varied evenby dividing the surface D into two or more surfaces as shown in FIG. 9(see a fourth example), as well as by making the surface D be anon-parallel surface. Also in this case, the interference modes arechanged to eliminate the ultrasonic wave interference mode. In addition,by attaching the sound absorbing material to a particular portion tosuppress only a particular interference mode in a focused manner, theamount of the sound absorbing material necessary for efficientinterference mode suppression can be reduced.

In the above-described acousto-optic modulator of the embodiment, sincethe surfaces of the acousto-optic medium are formed nonparallel to eachother to suppress the ultrasonic wave interference mode, the stabilityof intensity of the light generated during frequency modulation by theacousto-optic modulator can be remarkably enhanced without using anexpensive sound absorbing material.

EXAMPLES

Four examples will now be described.

First Example

As shown in FIG. 1, a piezoelectric transducer 110 is attached to anacousto-optic medium 100 formed of crystal of tellurium dioxide, and ahigh-frequency circuit 130 is attached to the piezoelectric transducer110 by copper wires 120. At this time, the acousto-optic medium 100 isprocessed into the configuration shown in FIG. 2, with θ=8°, φ=20° andθ′=23°. Further, the surface G of the acousto-optic medium 100 is coatedwith a sound absorbing material 210, such as silver paste, as is shownin FIG. 2.

The high-frequency circuit 130 applies a high-frequency signal forfrequency modulation to the acousto-optic transducer 110 via the copperwires 120. When incident light 150 is input to the acousto-opticmodulator with a Bragg angle, diffracted light 151 and transmitted light152 are emitted from the modulator. The diffracted light 151 is thefrequency-modulated light of the incident light 150. The intensitystability of the diffracted light 151 is significantly higher than inthe case of θ′=0°.

Second Example

As shown in FIG. 1, a piezoelectric transducer 110 is attached to anacousto-optic medium 100 formed of tellurium dioxide crystal, and ahigh-frequency circuit 130 is attached to the piezoelectric transducer110 by copper wires 120. At this time, the acousto-optic medium 100 isprocessed into the configuration shown in FIG. 5, with φ=20° and φ′=15°.Further, the surface G of the acousto-optic medium 100 is coated with asound absorbing material 210, such as silver paste, as is shown in FIG.5. The high-frequency circuit 130 applies a high-frequency signal forfrequency modulation to the acousto-optic transducer 110 via the copperwires 120. When incident light 150 is input to the acousto-opticmodulator with a Bragg angle, diffracted light 151 and transmitted light152 are emitted from the modulator. The diffracted light 151 is thefrequency-modulated light of the incident light 150.

When the angle defined between the surface H opposing the surface G andthe surface C is set to π/2+φ′ and the angle defined between the surfaceG and the surface D is set to π/2−φ as shown FIG. 6, if a mode in whichlight perpendicularly enters the surface C does not exist, nφ′≠π/2−2φ.In contrast, in the case of nφ′≠π/2−2φ, the above-mentioned interferencemode is suppressed, and the intensity stability of the diffracted light151 is significantly higher than in the case of φ′=0°.

Third Example

As shown in FIG. 1, a piezoelectric transducer 110 is attached to anacousto-optic medium 100 formed of tellurium dioxide crystal, and ahigh-frequency circuit 130 is attached to the piezoelectric transducer110 by copper wires 120. At this time, the acousto-optic medium 100 isprocessed into the configuration shown in FIG. 5, with φ=20° and φ″=35°.Further, the surface G of the acousto-optic medium 100 is coated with asound absorbing material 210, such as silver paste, as is shown in FIG.7. The high-frequency circuit 130 applies a high-frequency signal forfrequency modulation to the acousto-optic transducer 110 via the copperwires 120. When incident light 150 is input to the acousto-opticmodulator with a Bragg angle, diffracted light 151 and transmitted light152 are emitted from the modulator. The diffracted light 151 is thefrequency-modulated light of the incident light 150.

When the angle defined between the surface G and the surface C is set toπ/2−φ″ and the angle defined between the surface G and the surface D isset to π/2−φ as shown FIG. 8, if a mode in which light perpendicularlyenters the surface C does not exist, nφ″≠|π/2−2φ| and φ″≠−φ. Incontrast, in the case of nφ″≠|π/2−2φ| and φ″≠−φ, the above-mentionedinterference mode is suppressed, and the intensity stability of thediffracted light 151 is significantly higher than in the case of φ″=0°.

Fourth Example

As shown in FIG. 1, a piezoelectric transducer 110 is attached to anacousto-optic medium 100 formed of tellurium dioxide crystal, and ahigh-frequency circuit 130 is attached to the piezoelectric transducer110 by copper wires 120. At this time, the acousto-optic medium 100 isprocessed to have two surfaces D′₁ and D′₂ opposing the piezoelectrictransducer 110 as shown in FIG. 9. Further, the length of the side atwhich a surface H′ opposing a surface G′ contacts the surface D′₁ is setto L₁, and the length of the side at which the surface H′ contacts thesurface D′₂ is set to L₂ different from L₁, with θ₁=20° and θ₂=35°, asshown in FIG. 9. The high-frequency circuit 130 applies a high-frequencysignal for frequency modulation to the acousto-optic transducer 110 viathe copper wires 120. When incident light 150 is input to theacousto-optic modulator with a Bragg angle, diffracted light 151 andtransmitted light 152 are emitted from the modulator. The diffractedlight 151 is the frequency-modulated light of the incident light 150.The intensity stability of the resultant diffracted light 151 issignificantly higher than in the case of FIG. 2 where only one surfaceopposes the piezoelectric transducer 110. Although in the fourthexample, two surfaces oppose the piezoelectric transducer 110, the sameadvantage can be obtained if three or more surfaces are opposed to thepiezoelectric transducer.

The above-described configurations and locations are merely examples,and other locations and configurations that provide the same advantageas the above may be employed. Further, the material of the acousto-opticmedium 100 described above is also merely an example, and a differentmaterial, such as lead molybdate, exhibiting a similar acousto-opticeffect can be used. Yet further, the sound absorbing material 210 is notlimited to the silver paste, but may be a different material. It issufficient if the material has the same acoustic impedance as theacousto-optic medium and a high ultrasonic wave attenuation factor. Forinstance, the material may be a silver paste mixed with iron powder.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. An acousto-optic modulator comprising: anacousto-optic medium having a configuration of a polyhedron with notless than seven surfaces; and a piezoelectric transducer provided on aseventh surface (C′) of the acousto-optic medium, wherein the seventhsurface (C′) has four respective sides shared by twelfth, thirteenth,tenth, and eleventh surfaces (E′, F′, G′, and H′), four angles definedbetween the seventh surface (C′) and the twelfth, thirteenth, tenth, andeleventh surfaces (E′, F′, G′ and H′) are each ninety degrees, one ofthe twelfth and thirteenth surfaces (E′ and F′) serves as a light inputsurface, and the other one of the twelfth and thirteenth surfaces (E′and F′) serves as a light output surface, and a number of surfaces(D′_(i)) opposing the seventh surface (C′) and being formed nonparallelto the seventh surface (C′) is not less than two, i being an integer notless than two.
 2. The element according to claim 1, wherein theacousto-optic medium further has a configuration of a polyhedron withseven surfaces and has eighth and ninth surfaces (D′₁ and D′₂) opposingthe seventh surface (C′), the tenth surface (G′) coated with a soundabsorbing material, and the eleventh surface (H′) opposing the tenthsurface (G′); and a side shared by the tenth and eighth surfaces (G′ andD′₁) and a side shared by the tenth and ninth surfaces (G′ and D′₂) havedifferent lengths.